1. Field of the Invention
This invention relates to solid-state array scanners for sensing color images and monochromatic images. More particularly, the invention relates to operating such a scanner in a high-resolution monochromatic mode when scanning black/white images such as text documents.
2. Description of Related Art
Solid-state array scanners for scanning color images are well known. These scanners require a set of solid-state photosensitive devices to sense the color of a given picture element (pixel) in the image being scanned. There is a solid-state photosensor assigned to each of the primary colors of a picture element. Typically, this is accomplished by using a cyan, green, yellow or magenta filter with each solid-state photosensor. Other solid-state scanners have used sets of white, red, green and blue filtered solid-state photosensors per color pixel, or simply sets of red, green and blue filtered solid-state photosensors per color pixel.
The solid-state photosensors are usually arranged in a matrix with a plurality of sets of three or four photosensors per set. Each photosensor in a set produces a color-filtered pixel signal. Each color-filtered (red, green or blue, for example) pixel signal from one of the solid-state photosensors in the set is processed outside the array with the other color-filtered pixel signals from photosensors in the set to produce a color pixel signal. This pixel signal is treated as if it originated at each solid-state photosensor in the set. If each solid-state photosensor could be used to produce a distinct monochrome pixel signal when text documents are being scanned, the resolution of the array scanner could be increased by a factor equal to the number of photosensitive devices used to generate a color pixel signal; i.e., typically a factor of 3 or 4.
Exemplary of solid-state image scanners are U.S. Pat. Nos.: 4,117,510 issued to Ohta et al, 4,930,006 issued to Murayama et al, 5,170,249 issued to Ohtsubo et al and 5,307,159 issued to Hieda. Each of these patents teaches a different technique for scanning a matrix of solid-state photosensors with color filters placed over each of the solid state photosensors. Also, the resolution for each of the scanners is set by the number of solid state photosensors grouped to produced the color signals for a color pixel or pel (picture element).
U.S. Pat. No. 4,876,590 issued to Parulski does teach using different sets of color sensors in an array scanner. In normal color scanning, all color sensors are used. In high speed scanning, only the color sensors for a single color are used. Thus, the majority of the sensors go unused during monochromatic scanning.
U.S. Pat. No. 4,658,287 issued to Chen uses each solid-state sensor for each black/white pixel and a set of red, green and blue solid-state sensors for each color pixel. However, Chen removes his color filter when switching from color scanning to black/white scanning. This is not practical in a solid-state scanner where the color filters are integrated into the structure of the scanner array.
To date, there is no solid-state scanner that has an integrated structure with color filters in the scanner array that can be switched from color resolution using a set of solid-state sensors for each color pixel to a higher black/white resolution using each solid-state sensor for each black/white pixel.